The salient advantage of wireless telecommunications over wireline telecommunications is that the user of the wireless terminal is afforded the opportunity to use his or her terminal anywhere. On the other hand, the salient disadvantage of wireless telecommunications lies in that fact that because the user is mobile, an interested party might not be able to readily ascertain the location of the user.
Such interested parties might include both the user of the wireless terminal and a remote party. There are a variety of reasons why the user of a wireless terminal might be interested in knowing his or her location. For example, the user might be interested in telling a remote party where he or she is or, alternatively, the user might seek advice in navigation.
In addition, there are a variety of reasons why a remote party might be interested in knowing the location of the user. For example, the recipient of an E 9-1-1 emergency call from a wireless terminal might be interested in knowing the location of the wireless terminal so that emergency services vehicles can be dispatched to that location.
There are many techniques in the prior art for estimating the location of a wireless terminal. In accordance with some techniques, the location of a wireless terminal is estimated, at least in part, from measurements that are reported by the wireless terminal. The reported measurements are of signals measured by the wireless terminal that are transmitted by one or more base stations and, in some cases, by Global Positioning System (GPS) satellites. Some techniques rely on signal-strength measurements, while some other techniques rely on time-based measurements, while still some other techniques rely on other types of measurements. In order for these estimation techniques to work, at least some of the transmitted signals have to be strong enough to allow for accurate measurement by the wireless terminal and for reliable processing by the particular technique.
In some operating scenarios, conditions exist that might be insufficiently or incorrectly characterized by some entity—for example, by the service provider that controls the infrastructure providing service to the wireless terminal. One such condition is propagation time, in which the base station or wireless terminal measures the round-trip time (RTT), or equivalent, of the signal being measured. Here, the service provider might attempt to correct for the component of the propagation time attributed to the base station equipment, by subtracting off the electrical delay of the equipment from the RTT measurement prior to providing the measurement to an application that uses it to locate the wireless terminal. This type of error, as well as other errors, can impair the performance of at least some location estimation techniques in the prior art.